A cross-section of planet Earth

Geologists know of Earth structure by studying earthquake waves bouncing around inside. Some waves go through solids and liquids, but other kinds do not pass through liquids.

Our planet contains three main layers. The central 10% of the volume is the core, made of iron-nickel alloy. Its density is about 7 g/cc (grams per cubic centimeter), its thickness is 1,990 miles out from the Earth's center. A cubic foot of core metal weighs about 440 pounds, so the whole core weighs 2.1 x 1,024 pounds or about 1,021 tons. (Archimedes, weep.) The core's outer part is liquid (~peanut butter), its inner part is solid due to the immense pressures acting upon it and its temperature of 8,000° F is about the same as the surface of the sun. Heat is generated within the core (due to radioactive decay or left over from accretion) and rises as hot convection cells in the outer molten portion, just like boiling water on your stove, in some mysterious way so as to produce the dipole magnetic field we measure at the surface. Its amazing that some birds use the magnetic field for direction-finding during migration -- those with tiny magnets strapped to their heads get lost. This energy field is important because it deflects cosmic and solar subatomic particles away from the surface. Occasional intense doses of incoming particles cause the northern lights. The bigger problem with producing a working model of the magnetic field is that the field collapses 2-6 times each million years, spaced very irregularly, and regenerates with a total pole shift north-to-south, taking some 2,000 years to complete. This strange factoid is recorded in the magnetic properties of seafloor basalt masses that issues out of ocean spreading centers and 'freeze in' the current field, and so we record magnetic stripes on the seabeds parallel to the crack zones that produce the masses. There is some questionable evidence that the core actually rotates faster than the mantle above -- but I can't see how this can be. But then I never understood the Earth very well.

The main bulk of planet Earth is the mantle, making up nearly 90% of the planetary mass. It is dense rock material called peridotite, made mostly of the minerals olivine, pyroxene, spinel, magnetite and plagioclase feldspar (Figure 43). It has the same minerals as stony meteorites which we figure with some finesse came from the innards of busted planets--the story told in meteorites. The mantle's density is about 3.5 g/cc, its thickness is 1,830 miles. Its structure is certainly complex and unknown. It generates internal heat by radioactive decay and will continue the production for some billions of years more. The heat builds up in the upper regions enough to melt rocks into the jelly layer mentioned below in Figure 3; this heat produces magma masses that rises as convection cells that causes all kinds of disruptions on the surface, and accounts for drifting and mashing of continental 'plates' and volcanism. We know a bit about the mantle rocks because small chunks of them (called xenoliths) get blown out of basalt volcanoes on land. There are three basalt fields in Arizona that contain mantle xenoliths including the San Bernardino field near Portal. Mankind has yet to drill a hole through the continental crust layer -- we go down barely five miles, one-fifth of the way.

The outer thin Earth layer is the cold solid rocky crust, of two very different kinds. Oceanic crust underlies all the major ocean areas (about 70% of the surface), and is made of the volcanic rock basalt (pyroxene, olivine, spinel, plagioclase feldspar, magnetite) (Figure 3). Ocean crust is about 2 miles thick and rides along on the upper mantle layer. Oceanic basalt blows out of the Hawaiian volcanoes. Continental crust is about 25 to 40 miles thick and is made of a mix of granite and schist, covered over by lots of interesting youthful layers--limes, sands, muds, volcanoes. These crustal rocks are made of two kinds of feldspar, quartz, micas, hornblende, and a little magnetite. Ocean crust is more dense -- about 3.3 g/cc, while continental crust is about 2.8 g/cc. But due to its far greater thickness and lighter weight, continental crust floats higher upon the mantle and so, by chance, continents float around like big battleships above sea level. What would the world look like if water covered all the land?


It's amazing that there is evidence from growth patterns in old marine fossils that Earth's day length is slowing down due to tidal friction losses. Projected back to the beginning days, the day length was some 4-6 hours, and the moon in much closer orbit, causing tides in the shallow oceans to flood over the tiny islands every few hours.

Continental drift is a fact. We do satellite GPS measurements of the rate of drift. The Atlantic Ocean is wider each year by 4-8 inches, and the Pacific narrower by about the same amount. Thus, North America seems destined to collide with Asia, making San Francisco a suburb of Beijing and Hawaii a grease spot in the middle. At 4 inches per year, it'll take about 126 million years. Meanwhile, Africa is being dismantled -- ripped apart by forces underneath so that soon all land east of the rift zone will be a new large island next to Mozambique in the Indian Ocean. North America too has a buried but active rift pulling it apart under the Mississippi River. Ten million years ago (10 m.y. ago) Africa collided with Europe with enough force to push up the Alps, then it backed off, and so the Mediterranean hole flooded full of water. Still, the overall process of volcanism over long times continues to produce more continental crust -- there are more square miles of continental land now than ever before. As I describe below, North America, like all continents, is a 'composite' land, made of many collisions of smaller land masses that became super-glued together. The rock called schist acts as the gluing medium, made of old coastal sediments that got squeezed and nearly melted as continents collided. So the ultimate 'basement rocks' of all regions is a mix of granite islands held together by bands of schist, plus oddball other kinds.

Figure 1 cross-section shows the continent in three colors, implying three older land masses that have collided and fused together. The figure also labels two important surface events that are ongoing -- subduction and seafloor spreading. Subduction zones are where a continent pushes against a seafloor and shoves the seafloor down under. These zones are where many earthquakes originate such as off Chile. Spreading centers in the oceans or a 'rift zone' on land are major cracks that draw open and where basalt magma pours out and solidifies. The figure shows red and white striping in the seafloor crust -- which represents the periodical reversing of the Earth's magnetic field that is 'frozen' into the magnetic minerals of the newly cooled basalt masses along the spreading centers, so that we can map magnetic stripe patterns on the seabeds. In fact this revelation of magnetic striping on the seabeds discovered in the 1950s was major evidence for continental drifting. The third kind of plate boundaries are transform faults, like the San Andreas fault. These are sideways-sliding faults that commonly tear at edges of continents behind subduction zones. Another important feature on Earth are 'hot spots,' which are long-lasting and stable point sources of extreme heat formed where three upward-welling mantle convection cells ajoin, to supply continued heat to a single point in the crust, and causing continued active volcanism at the surface at that spot. Both Hawaii and Yellowstone are hot spots. Some geologists count about 40 hot spots on Earth. The spots migrate sideways as the crust slowly moves over the mantle spot, and so the Yellowstone volcanic center is migrating slowly towards the NE.

There is an unexplained monstrous cycle on Earth first indicated by the Canadian geologist J. Tuzo Wilson in the 1960s. He called attention to how continents seem to break apart, and then all congeal together into supercontinents, which are stable for a time, but then they break up again. We have had probably three such 'super cycles' through all of geo-time. In his honor this cycling is called a Wilson cycle. It must be a manifestation of big things going on in the mantle, like the 'pulses of the Earth' described by the geologist Umbgrove a long time ago.

Section 2. Geological Time Scale

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